1. Field of the Invention
This invention relates to switching circuits and more particularly to pulse energy suppression during the turnoff transition of high power switching transistor circuits.
2. Description of the Prior Art
Power transistors are widely used in switching mode in regulators, inverters and the like since operation of a transistor between full conduction and cutoff usually results in minimal power dissipation. However, there is necessarily some power dissipation in the transistor during its transition in the active region between these two states, when current is flowing across a substantial potential gradient within the transistor. In transformer-coupled transistor switching regulators wherein energy must be dissipated during reset of the transformer, clamp circuits may be provided across the primary of the transformer to absorb most of the leakage flux and magnetizing energy in the transformer and to limit the voltage excursion impressed upon the transistor as it suddenly interrupts the current flowing through the transformer primary. However, such clamps usually take some time to become operative and in any event do not fully isolate the switching transistor from high potential during its turnoff transition since potential is needed for the transformer resetting action. Typical examples of transistor switching regulators of this general kind are seen in U.S. Pat. Nos. 3,697,852 and 3,859,590, both assigned to the assignee of this application.
The dynamics of turnoff and turnon transitions of a power transistor have been studied extensively. One example of a reference which summarizes some of the techniques which have been suggested in response to this problem is an article by E. T. Calkin and D. H. Hamilton entitled "Current Techniques for Improving the Switching Loci of Transistor Switches in Switching Regulators", 1972 IEEE Industry Applications Society Conference Record, pages 447-484. In that paper, the use of a capacitor to clamp and thus retard the rise of the collector potential of a switching transistor, so as to reduce turnoff dissipation, it shown. However, the transient across the capacitor is a function of its charge and, therefore, the clamping action is imperfect and the switching dissipation reduction is only partial.